Said H M, Derweesh I
Department of Medicine, University of California School of Medicine, Irvine 92717.
Am J Physiol. 1991 Jul;261(1 Pt 2):R94-7. doi: 10.1152/ajpregu.1991.261.1.R94.
Simple diffusion has been reported as the mechanism of biotin transport in rabbit intestine. In this study, we reevaluated this concept by examining biotin transport in rabbit intestine using optimal experimental conditions and a well-established brush-border membrane vesicles (BBMV) technique. Uptake of biotin by rabbit intestinal BBMV was found by an osmolarity study to be mostly the result of transport of the vitamin into an osmotically sensitive intravesicular space with little binding to membrane surfaces. Biotin transport in rabbit intestinal BBMV was 1) Na+ gradient dependent (out greater than in) with a clear "overshoot" phenomenon, indicating the accumulation of the substrate against a concentration gradient; 2) initial rate of biotin transport by the Na+ gradient-dependent component was saturable as a function of substrate concentration with apparent Km and maximum velocity (Vmax) values of 6.7 microM and 10.7 pmol.mg protein-1 x 10 s-1, respectively; 3) inhibited by high concentrations of unlabeled biotin and its related compounds desthiobiotin and thioctic acid in the presence, but not absence, of a Na+ gradient; and 4) not affected by inducing a relatively positive or negative intravesicular space with the use of valinomycin-induced K+ diffusion potential. These findings indicate that the biotin transport mechanism in rabbit intestine is carrier mediated in nature. Furthermore, this mechanism is Na+ gradient dependent, capable of accumulating the substrate against a concentration gradient and transport the vitamin via an electroneutral process.
已有报道称，简单扩散是生物素在兔肠道中运输的机制。在本研究中，我们通过使用最佳实验条件和成熟的刷状缘膜囊泡（BBMV）技术检测兔肠道中的生物素运输，重新评估了这一概念。通过渗透压研究发现，兔肠道BBMV对生物素的摄取主要是维生素运输到对渗透压敏感的囊泡内空间的结果，与膜表面的结合很少。兔肠道BBMV中的生物素运输具有以下特点：1）依赖Na+梯度（胞外大于胞内），并有明显的“过冲”现象，表明底物逆浓度梯度积累；2）Na+梯度依赖成分介导的生物素运输初始速率随底物浓度呈饱和状态，表观Km和最大速度（Vmax）值分别为6.7 μM和10.7 pmol·mg蛋白-1×10 s-1；3）在有Na+梯度存在而非不存在时，高浓度未标记生物素及其相关化合物脱硫生物素和硫辛酸可抑制其运输；4）使用缬氨霉素诱导的K+扩散电位诱导相对正或负的囊泡内空间对其无影响。这些发现表明，兔肠道中的生物素运输机制本质上是载体介导的。此外，该机制依赖Na+梯度，能够逆浓度梯度积累底物，并通过电中性过程运输维生素。
